This invention relates generally to radio frequency circuits and more particularly to digital control of radio frequency circuits.
As it is known in the art, monolithic microwave integrated circuits are used in a variety of applications. Such circuits, including amplifiers, switches, phase shifters, attenuators, and the like often require control signals to be fed to the circuits to switch amplifiers on and off, change the state of a switch, vary a phase shift imparted to a signal propagating through a phase shifter, or vary the amplitude of a signal fed through an attenuator, for example.
One particular application for these digitally controlled circuits is such circuits arranged in a transmit/receive (T/R) module for use in a phased array antenna system. A phased array antenna includes a plurality of such T/R modules arranged in an array with each module having the capability to impart to a signal a selected differential phase and/or amplitude characteristic. In a transmit mode the plurality of T/R modules are fed a common transmit signal through a common feed network to provide a plurality of signal portions of said common signal each of which is acted upon by the T/R modules to produce a plurality of transmit signals. This plurality of signals having selected phase and amplitude characteristics is fed to a corresponding plurality of radiating elements and is radiated to provide beams of directed and generally collimated electromagnetic energy. The T/R modules are used to form such beams and to electronically steer such beams by varying the phase and amplitude characteristics of the signals radiated therefrom. A similar arrangement occurs during a receive mode of operation.
There are many arrangements for a T/R modules. A common configuration of a T/R module includes a reciprocal phase shifter and an optional attenuator disposed in a common path. A pair of switching circuits and a pair of amplifiers are arranged to provide a pair of switchable signal amplication paths between a radiating element and the common path including the phase shifter. One of the amplifiers is a high-power amplifier used as a transmit amplifier, whereas the other amplifier is a low-noise amplifier used as a receive amplifier. The switches are provided to switch a signal from the phase shifter to the transmit amplifier and out to a radiating element during a transmit mode of operation and to switch the signal path to couple a receive signal from the radiating element through the receive amplifier and to the phase shifter during a receive mode of operation.
A common type of phase shifter employed in such a T/R module is a digitally controlled phase shifter, which in response to a control word imparts a selected incremental differential phase shift to a signal propagating therethrough. That is, a common configuration of a digitally controlled phase shifter includes a plurality of switchable phase sections which are controlled to provide the selectable phase shift characteristic by feeding a parallel digital word provided along a corresponding plurality of control lines to the phase shifter. Similarly, attenuators are also often employed in such circuits to provide a variable amplitude characteristic to such signal. The attenuator would typically include digitally switchable or analog controlled segments, which are also controlled by a digital word fed along a plurality of control lines.
Since transmit/receive modules are operated in the microwave range of frequencies and higher (e.g., millimeter wave) size and complexity are important considerations. Moreover, packaging of such a circuit is also a major consideration, particularly at very high microwave frequencies, since it is generally required in the phase array to space the radiating elements less than one-half of a wavelength apart.
Heretofore, control signals for the analog integrated circuits have been provided from separate digital integrated circuits mounted in a common package with the analog circuits. The digital integrated circuits typically will provide a plurality of control lines (eg. six to twelve lines to control a 6-bit digital phase shifter) which are connected to the analog circuit by wire bonds. That is, the digital circuits are mounted within the T/R module but are separated from the monolithic microwave circuits which they are intended to control. The control lines from the separate digital circuits are generally hand bonded to bonding pads on the analog monolithic microwave integrated circuits. Thus, while this arrangement provides such control signals to the analog monolithic microwave integrated circuit it also increases the number of external connections made to the circuit which increases the cost of packaging the T/R circuits as well as the size of the module. Also, the presence of the many bonding wires often presents difficult problems associated with proper r.f. and digital layout of the circuits within the module. For example, to control a six-bit digital phase shifter would require up to six pairs of digital control lines or a total of 12 lines to select various phase shift increments with each pair of lines carrying the true logic level and complement level of bit of the parallel control signal. A corresponding number of lines would also be required to hook-up the other elements, such as the variable attenuators, the switches, and switched amplifiers.
In accordance with the invention, a variable phase shifter is provided, such phase shifter having a semiconductor substrate, means supported on said semiconductor substrate and responsive to a control signal fed to said means, for providing a selectable phase shift to a radio frequency signal propagating therethrough, such phase shift being selected in accordance with the control signal, and digital control means supported on said semiconductor substrate and fed by a serial digital data stream for converting said serial digital data stream into a stream of signals to provide said control signal to the phase shift means.
In one embodiment of the invention, the phase shifter is included in a transmit and receive circuit (T/R circuit). More particularly, the T/R circuit includes a semiconductor substrate, means supported on said semiconductor substrate and responsive to a control signal fed to said means, for providing a selectable phase shift to a radio frequency signal propagating therethrough, said means further comprising digital control means supported on said semiconductor substrate and fed by a first serial digital data stream for converting said serial digital data stream into a stream of parallel signals to provide the control signal; and means for switching a radio frequency signal between two paths and for providing amplification of said signal in each of said paths with said switching means feeding said radio frequency signal to said phase shift means in a first mode of operation and with said phase shifting means feeding said radio frequency signal to said switching means in a second mode of operation of said T/R circuit.
In accordance with yet another feature of the invention, a digitally controlled phase shifter is provided. The phase shifter included a semiconductor substrate, means supported on said semiconductor substrate and responsive to a control signal fed to said means, for providing a selectable phase shift to a radio frequency signal propagating therethrough, and digital control means supported on said semiconductor substrate and fed by a serial digital data stream for converting said serial digital data stream into a stream of signals to provide said control signal to the phase shift means, said digital control means including digital to analog converter means fed by said stream of signals and disposed on said substrate for converting said stream of signals into the control signal.
In accordance with still another feature of the invention, a transmit and receive apparatus is provided. The apparatus include a semiconductor substrate, a circuit on said substrate for receiving a radio frequency signal and for changing electrical characteristics of such radio frequency signal as such signal passes through such circuit selectively in accordance with a digital signal fed to such circuit, such digital signal comprising a plurality of bits, a shift register on the substrate for serially receiving the plurality of bits and for storing such bits in response to clock pulses fed to the register, and a latch on the substrate for receiving in parallel, in response to a latch signal, the serially received bits stored in the shift register, the latch providing the received bits from the shift register and presenting such received bits in parallel to said circuit as said digital signal.